1. Field of the Invention
The present invention relates to a hydraulic braking system for use in an automotive vehicle, and more particularly to a hydraulic braking system having a hydraulic booster for actuating a master cylinder in response to operation of a brake operating member with a hydraulic power pressure supplied from a power source, wherein an anti-skid control operation is performed with the hydraulic power pressure applied to wheel brake cylinders.
2. Description of the Prior Art
In conventional service braking systems for an automotive vehicle, there are provided a plurality of hydraulic circuits connecting a master cylinder with wheel brake cylinders mounted on road wheels, so that when one of the hydraulic circuits is broken, braking operation is performed by the rest of the hydraulic circuits. In general, a tandem master cylinder is used in a conventional dual circuits system. In order to reduce the force required to operate a brake operating member, for example a depressing force applied on a brake pedal in braking operation, the hydraulic braking system is provided with a servo unit which is referred as a servo or a booster and which utilizes compressed air, intake manifold vacuum (for a vacuum booster), or hydraulic pressure (for a hydraulic booster) as a power source. The hydraulic booster is a booster which actuates the master cylinder by a hydraulic power pressure supplied from the power source in response to depression of the brake pedal.
As for the hydraulic braking system having the hydraulic booster, it has been proposed to employ the hydraulic booster as a pressure source for the anti-skid control device, whereby a hydraulic pressure in each of the wheel brake cylinders is controlled in response to a slip rate of each of the road wheels to thereby prevent the road wheels from locking. In this hydraulic braking system, the normal braking operation is carried out by a hydraulic braking pressure generated in a pressure chamber of the master cylinder (master cylinder pressure), whereas the anti-skid control operation is performed on the basis of the hydraulic power pressure. Accordingly, even if the hydraulic pressure in the wheel brake cylinder is increased or decreased in the anti-skid control operation, the brake operating member will not receive any reaction due to the change of the hydraulic pressure in the wheel brake cylinder.
However, in the case where the hydraulic power pressure applied to the wheel brake cylinder is insufficient, such as the case where a control valve provided in the hydraulic booster for controlling the hydraulic power pressure introduced thereinto from the power source is jammed, if the anti-skid control operation is performed, the braking force applied to the road wheels will be insufficient.